1. Field of the Invention
The present invention relates to an optical pickup head device that is used in an apparatus for recording, reproducing or erasing information on an optical recording medium and an information recording/reproducing apparatus. The present invention also relates to an information recording method that employs the optical pickup head device.
2. Description of the Related Art
An optical memory technique that uses optical disks having pit patterns as high-density large-capacity recording media is finding wide application, e.g., to digital audio disks, video disks, document file disks, and data files. In recent years, high-density large-capacity optical disks, called DVD, have been put to practical use and attracted considerable attention as information media that can handle mass information like animation. The DVD optical disks are recorded/reproduced with a so-called red semiconductor laser that emits a laser beam having a wavelength of about 650 nm.
FIG. 10 shows the configuration of a general optical system used in an optical pickup head device of an optical disk system that can perform recording and reproduction. A semiconductor laser source 1, i.e., a light source, emits a linearly polarized divergent beam 700 having a wavelength λ1 of 650 nm. The beam 700 emitted from the semiconductor laser 1 enters a diffraction grating 60. The diffraction grating 60 divides the beam 700 into three beams: a zero-order diffracted light beam and ±first-order diffracted light beams. The zero-order diffracted light beam is a main beam 700a for recording/reproducing information. The ±first-order diffracted light beams are two sub-beams 700b, 700c used for differential push-pull (DPP), with which a tracking error signal can be detected stably. To avoid unnecessary recording by the sub-beams, the ratio of diffraction efficiency of the zero-order diffracted light beam 700a to each of the first-order diffracted light beams 700b, 700c is generally about 20:1. The three beams 700a to 700c generated in the diffraction grating 60 pass through a polarizing beam splitter 52 and enter a collimator lens 53 having a focal length of 20 mm. The collimator lens 53 converts the beams into parallel light. The beams 700a to 700c thus collimated pass through a quarter-wave plate 54, where the beams are converted into circularly polarized light. Then, the beams are converted into convergent beams with an objective lens 56 having a focal length of 3 mm, pass through a transparent substrate 41a of an optical recording medium 41, and are focused on an information recording plane 41b. The aperture of the objective lens 56 is limited by an aperture 55 so that the NA is 0.6. The transparent substrate 41a has a thickness of 0.6 mm.
FIG. 11A is a front view schematically showing the diffraction grating 60, and FIG. 11B is a cross-sectional side view of the diffraction grating 60. A Y-direction is parallel to a tangent to the track on the optical recording medium 41 and an X-direction is perpendicular thereto. Straight grating patterns are formed on the diffraction grating 60 at an equal period of Pt. The grating depth d0 is set so that the ratio of light amount of the beam 700a to each of the beams 700b, 700c is 20:1. An angle θ between a spatial frequency axis 60d of the diffraction grating 60 and the Y-axis is determined by the positional relationship between the tracks on the information recording plane 41b and the focused beams 700a to 700c, and generally is in the range of about 1 to 2 degrees.
FIG. 12 shows the relationship between the beams 700a to 700c on the information recording plane 41b and the tracks. The optical recording medium 41 is provided with continuous grooves that serve as the tracks. The track period Tp is 0.74 μm. The beams are arranged so that when the main beam 700a is positioned on a track, each of the sub-beams 700b, 700c is positioned between tracks. In other words, a distance L between the main beam 700a and the sub-beam 700b or 700c in the direction perpendicular to the tracks is 0.37 μm.
The beams 700a to 700c reflected from the information recording plane 41b pass through the objective lens 56 and enter the quarter-wave plate 54, where the beams are converted into linearly polarized light that differs by 90 degrees from the light traveling from the semiconductor laser 1 to the optical recording medium 41. Then, the beams are converted into convergent beams by passing through the collimator lens 53 and reflected from the polarizing beam splitter 52. The beams 700a to 700c reflected from the polarizing beam splitter 52 pass through a cylindrical lens 57 and enter a photodetector 31. The transmission of beams 700a to 700c through the cylindrical lens 57 imparts astigmatism to the beams. The photodetector 31 includes eight light receiving portions 31a to 31h. The light receiving portions 31a to 31d receive the beam 700a, the light receiving portions 31e, 31f receive the beam 700b, and the light receiving portions 31g, 31h receive the beam 700c. Each of the light receiving portions 31a to 31h outputs a current signal that corresponds to the amount of light received.
The output signals of the light receiving portions 31a to 31d for receiving the main beam 700a are used to generate a focusing error signal with an astigmatism method, a tracking error signal with a phase-difference method, and a tracking error signal with a push-pull method. When a disk having continuous grooves such as DVD-RW (registered trademark) is recorded/reproduced, the output signals of the light receiving portions 31e to 31h for receiving the sub-beams 700b, 700c are used together with the output signals of the light receiving portions 31a to 31d so as to generate a tracking error signal with a DPP method. The focusing error signal and the tracking error signal are amplified to a desired level and phase-compensated, and then sent to actuators 91, 92, thereby performing focusing control and tracking control.
In DVD, a two-layer disk that includes two information recording planes is standardized for read-only ROM disks. A conventional optical pickup head device can read information from the read-only two-layer disk without any problems by detecting a tracking error signal with the phase-difference method.
Research and development of an optical recording medium having two recordable information recording planes (hereinafter, referred to as a two-layer recording disk) has yielded significant results. Since no information is written in the two-layer recording disk in its initial state, a tracking error signal cannot be detected with the phase-difference method. Accordingly, like an optical recording medium having a single recordable information recording plane (hereinafter, referred to as a single-layer recording disk), the tracking error signal should be detected with the DPP method.
However, there is a problem of using the two-layer recording disk in a conventional device having the above configuration. Even if a tracking error signal is detected by the DPP method, it causes uncorrectable offset fluctuations when the objective lens follows tracking or the optical recording medium tilts. The reason for this is as follows. When information is recorded on one of the information recording planes (hereinafter, this information recording plane is referred to as a focusing plane), the beam focused on the focusing plane is partly reflected and partly transmitted by the focusing plane. The transmitted beam reaches the other information recording plane (hereinafter, this information recording plane is referred to as a non-focusing plane) in a defocused manner. The beam reflected from the non-focusing plane also enters the photodetector. However, this beam cannot be cancelled completely by the DPP method for detecting a tracking error signal because of aberration, a nonuniform light amount of the beam, or the like. Therefore, the amount of beam that is not cancelled varies when the objective lens follows tracking or the optical recording medium tilts, causing offset fluctuations in the tracking error signal. As a result, off-track occurs and erases some of the information recorded on the adjacent tracks during recording, so that information recorded on the optical recording medium cannot be read faithfully.